European Regulation (EU) 910/2014 lays down the rules for electronic identification and trust services for electronic transactions. Qualified Electronic Registered Delivery is one of the trust services included in the regulation, and it requires nonrepudiation of origin and reception together with the integrity of the data. This kind of services usually relies heavily on the use of trusted third parties. These entities are an obstacle to extend the use of protocols. In this paper, we use the blockchain-based technologies to provide, for the first time, two multiparty registered eDelivery schemes that reduce the involvement of trusted third parties compared with traditional approaches while satisfying the requirements of the norms created by the European Union for registered eDeliveries. Since confidentiality is not considered a compulsory property in the directive, we propose two protocols. The first one is well suited for those deliveries that do not require the confidentiality of the message or delivered data or even for those in which it is required that the message can be public and accessible to everybody. The second solution for multiparty registered eDelivery allows the message to be hidden to others than the receiver. We present the smart contracts for both proposals and also a complete analysis of their properties and performance.INDEX TERMS Blockchain, registered electronic delivery services, certified notifications, smart contract, confidentiality, fairness, cryptocurrencies, certified electronic mail.

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Recently several proposals of blockchain-based solutions for traditional e-commerce applications have been presented, taking advantage of the fact that blockchain is a technology that offers an immutable registry of data. Among these proposals we can find solutions for certified notifications, digital signature of contracts, escrow protocols, fair payments and registered deliveries. In order to execute fair exchanges, most solutions involve trusted third parties, known as TTP, supervising the exchanges in a way or another. Until now, two solutions have been presented for Registered electronic Delivery (eDelivery) services. This service allows a user to prove that he has sent some data to a set of receivers. These protocols differ in the properties achieved and also in the use of trusted third parties. The first protocol is a blockchainbased solution without TTP for the eDelivery of non-confidential data. The second protocol allows also the eDelivery of confidential data. However, this second proposal requires the involvement of a TTP in a nonmandatory resolution phase. In this paper we present a new protocol that achieves the best properties of the previous solutions at the same time. The new protocol doesn't require the involvement of a TTP at any moment while it allows the eDelivery of confidential data, satisfying the security requirements for this service.

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